Issue #23370: Fix off-by-one error for non-contiguous buffers.

Modules/_testcapimodule.c

@@ -424,6 +424,58 @@ test_to_contiguous(PyObject* self, PyObject *noargs)
     Py_RETURN_NONE;
 }
 
+static PyObject *
+test_from_contiguous(PyObject* self, PyObject *noargs)
+{
+    int data[9] = {-1,-1,-1,-1,-1,-1,-1,-1,-1};
+    int init[5] = {0, 1, 2, 3, 4};
+    Py_ssize_t itemsize = sizeof(int);
+    Py_ssize_t shape = 5;
+    Py_ssize_t strides = 2 * itemsize;
+    Py_buffer view = {
+        data,
+        NULL,
+        5 * itemsize,
+        itemsize,
+        1,
+        1,
+        NULL,
+        &shape,
+        &strides,
+        NULL,
+        {0, 0},
+        NULL
+    };
+    int *ptr;
+    int i;
+
+    PyBuffer_FromContiguous(&view, init, view.len, 'C');
+    ptr = view.buf;
+    for (i = 0; i < 5; i++) {
+        if (ptr[2*i] != i) {
+            PyErr_SetString(TestError,
+                "test_from_contiguous: incorrect result");
+            return NULL;
+        }
+    }
+
+    view.buf = &data[8];
+    view.strides[0] = -2 * itemsize;
+
+    PyBuffer_FromContiguous(&view, init, view.len, 'C');
+    ptr = view.buf;
+    for (i = 0; i < 5; i++) {
+        if (*(ptr-2*i) != i) {
+            PyErr_SetString(TestError,
+                "test_from_contiguous: incorrect result");
+            return NULL;
+        }
+    }
+
+    Py_RETURN_NONE;
+}
+
+
 /* Tests of PyLong_{As, From}{Unsigned,}Long(), and (#ifdef HAVE_LONG_LONG)
    PyLong_{As, From}{Unsigned,}LongLong().
 
@@ -1833,6 +1885,7 @@ static PyMethodDef TestMethods[] = {
     {"test_lazy_hash_inheritance",      (PyCFunction)test_lazy_hash_inheritance,METH_NOARGS},
     {"test_broken_memoryview",          (PyCFunction)test_broken_memoryview,METH_NOARGS},
     {"test_to_contiguous",      (PyCFunction)test_to_contiguous, METH_NOARGS},
+    {"test_from_contiguous",    (PyCFunction)test_from_contiguous, METH_NOARGS},
     {"test_long_api",           (PyCFunction)test_long_api,      METH_NOARGS},
     {"test_long_and_overflow", (PyCFunction)test_long_and_overflow,
      METH_NOARGS},

Objects/abstract.c

@@ -550,7 +550,7 @@ PyBuffer_FromContiguous(Py_buffer *view, void *buf, Py_ssize_t len, char fort)
 
     /* Otherwise a more elaborate scheme is needed */
 
-    /* XXX(nnorwitz): need to check for overflow! */
+    /* view->ndim <= 64 */
     indices = (Py_ssize_t *)PyMem_Malloc(sizeof(Py_ssize_t)*(view->ndim));
     if (indices == NULL) {
         PyErr_NoMemory();
@@ -572,10 +572,10 @@ PyBuffer_FromContiguous(Py_buffer *view, void *buf, Py_ssize_t len, char fort)
      */
     elements = len / view->itemsize;
     while (elements--) {
-        addone(view->ndim, indices, view->shape);
         ptr = PyBuffer_GetPointer(view, indices);
         memcpy(ptr, src, view->itemsize);
         src += view->itemsize;
+        addone(view->ndim, indices, view->shape);
     }
 
     PyMem_Free(indices);